In recent years, research and development into a magneto-optical memory element has been much more carried out than research and development into a read only optical memory element, such as a so-called compact disk, and the magneto-optical memory element now comes into practice. The magneto-optical memory element has a high capacity and whereon information can be recorded, reproduced and erased. Lately, a study of the recording system has been made to realize higher speed and higher density recording, and especially a recording method called overwriting has been focused. In the overwriting, recorded information can be rewritten directly without erasing, thereby permitting time required for recording to be shortened.
The following will explain the overwriting together with a conventional recording system.
In recording, first a perpendicular magnetized film that is a recording medium is initialized by applying a strong external magnetic field or the like so as to produce a uniform magnetization direction. To make the explanation easy, it is assumed in this example that the direction of magnetization is set in a predetermined direction, e.g. upwards in a direction crossing the film surface at a right angle. Next, a laser beam having a high output level is irradiated on a portion wherein information to be recorded so as to raise the temperature thereon to or exceed the Curie temperature or the vicinity of the magnetic compensation temperature of the recording medium. This method allows the coercive force on the recording portion to be substantially zero, so that the magnetization direction of the recording portion can be switched downwards by applying a magnetic field whose direction is downwards in a direction crossing the film surface at a right angle. When the irradiation of the laser beam is stopped, the temperature of the recording portion return to room temperature, which permits the switched magnetization direction to be fixed and information to be recorded thermomagnetically.
In reproduction, a laser beam having such a low level of output that it does not switch the magnetization direction due to the temperature rise is irradiated on the recording medium, and information is detected optically by the use of an effect that the direction of rotation of a polarization plane varies according to the magnetization direction.
There are two information rewriting systems for writing new information in portions storing other information. (1) A system in which a recording medium is initialized through erasing operation so that a uniform magnetization direction of the recording medium is produced again, and new information is then recorded thereon. (2) The overwriting system in which information is directly rewritten without erasing by improving a recording medium or an external magnetic field generating device.
In the system of (1), an erase head needs to be provided separately, or recording can be performed only after erasing if only one magnetic head is used. However, providing the erase head causes a manufacturing cost rise and the device to be larger in size. In the mean time, if a single magnetic head is employed like many conventional devices, the same time taken for recording is required for erasing, so a long time is needed for rewriting.
If improving the external magnetic field generating device relating to the ore writing system of (2) is adopted, i.e. the direction of an external magnetic field is switched from downwards to upwards or vice versa according to information to be recorded while keeping the laser beam to have a high output level, the recording medium does not need to be changed much from the conventional one, and therefore it seems to be the most effective way.
To achieve a high-density recording, if the direction of an external magnetization is switched at a very high frequency rate, for example about 10 MHz, a coil and a coil core of the external magnetic field generating device need to be miniaturized to a sufficient level. This causes the intensity of the generated magnetic field to be lowered and the magnetic field generating area to be smaller. Therefore, the magnetic head and the recording medium need to get closer each other sufficiently, i.e. specifically the head-to-medium separation ranges from several micrometers to some tens of micrometers. If a stationary type magnetic head is employed, it is difficult to achieve the above-mentioned small clearance between the magnetic head and the recording medium as the surface of the recording medium is vibrated, i.e. bumps and pits exist in a circumferential direction.
To counteract this, as shown in FIGS. 7(a)(b), a flying head 26 of a slider type which is capable of sliding over a magneto-optical disk may be used as an external magnetic field generating device. In order to make the flying head 26 fly above the magneto-optical disk surface, the flying head 26 comprises a slider section 26a provided with a magnetic field generating member 26b and is pressed towards the magneto-optical disk surface while being supported by a suspension 27. The magnetic field generating member 26b is composed of a coil and a coil core (not shown specifically), and the suspension 27 is made, for example, of a plate spring. The flying head 26 maintains a constant flying height by making a flying force exerted upwards due to the air flow between the slider section 26a and the magneto-optical disk caused by the spinning disk balance a depressing force exerted downwards by the suspension 27. The flying type magnetic head thus described is adopted in conventional hard disk system, and the flying height in the system is generally submicron order. In the mean time, in case the recording medium is a magneto-optical disk, dust tends to stick on the disk surface more frequently as the magneto-optical disk is transportable, and troubles such as a crash may occur when the head gets close to the disk excessively. Therefore, a flying height of 5 .mu.m to 15 .mu.m which is higher than a flying height in the hard disk system is needed for the magneto-optical disks.
In magneto-optical recording using the flying head 26, the flying head 26 maintains a constant flying height while a magneto-optical disk rotates at a constant speed, however it needs to be supported by some means when the disk starts/stops rotating and is in a static state. For supporting the disk, two methods are listed. (I) Improving the supporting mechanism of the flying head 26 so as to prevent the head from making contact with the magneto-optical disk when the disk is in a static state. (II) Making the flying head 26 slide over or come into contact with the magneto-optical disk when the disk starts/stops rotating and is in a static state.
There is a problem in method (I) that the supporting mechanism becomes extremely complicated. Method (II) called CSS (Contact Start and Stop) method is a common use method in the field of hard disk. In the CSS method, the ability of a magneto-optical disk surface to endure slides is a very important factor. In a conventional hard disk system, a lubricating layer having the excellent ability to endure slides is formed on a recording medium. The lubricating layer is formed by applying, for example fluorocarbon lubricating oil which is a liquid lubricant (usually perfluoropolyether) or PTFE (polytetrafluoroethylene) which is a solid lubricant onto the recording medium.
Additionally, in the CSS method, it is necessary to prevent data from being lost by head crash when the recording medium starts or stops spinning, i.e. when the flying head slides over the recording medium. In the hard disk system, therefore, a flying head is generally allowed to perform contacting and sliding action in an area for CSS which is located inside of the recording area of the disk. With a recording and reproducing device using a magneto-optical disk, it may also be preferable to allow a flying head to perform a sliding action in an area other than the recording area of the disk.
As shown in FIGS. 10(a)(b), a conventionally suggested magneto-optical disk comprises a transparent substrate 21, a recording medium layer 22, a protective resin layer 23, and a center hub 24.
Polycarbonate which can be mass-produced by the injection molding is widely used as the transparent substrate 21. The transparent substrate 21 is provided with guide tracks and guide address grooves 21a whereon track and address information is recorded so as to guide an optical beam used for recording and reproducing information to a given position. In the injection molding, the guide tracks and the guide address grooves 21a are formed by the use of a stamper installed in a metallic mold (not shown) when the transparent substrate 21 is formed. At this time, a stamper presser groove 21b is secondarily formed by a member for fixing the stamper in the metallic mold, so called a stamper presser.
The recording medium layer 22 is formed such that it covers the guide tracks and the guide address grooves 21a, and has multilayer structure (however, shown as a single layer in the figure), for example, a first transparent dielectric film, a thin film made of an alloy of rare earth elements and transition metals that is a magneto-optical recording medium, a second transparent dielectric film, a reflecting film are laminated in that order onto the transparent substrate 21.
The protective resin layer 23 protects the recording medium layer 22 from scratches, dust and oxidization. For materials as the protective resin layer 23, an ultraviolet hardening resin is widely used. The ultraviolet hardening resin has excellent waterproof properties and environment resistance, and the advantages of handling and processing time. The protective resin layer 23 is usually formed by the spin coating method. For example, first the ultraviolet hardening resin is applied to the recording medium layer 22 by spin coating, wherein the resin is dropped onto the outer edge of the stamper presser groove 21b so as to cover the recording medium layer 22. Next, ultraviolet light is irradiated on the resin to harden it.
The center hub 24 is a means for loading a magneto-optical disk into a rotating spindle of a recording and reproducing device. Since the guide tracks/guide address grooves are not coaxial with a center hole 21c of the transparent substrate 21 generally, the center hub 24 absorbs this during the loading. The center hub 24 has a guide hole 24a which is used for loading the disk into the rotating spindle 25 (see FIG. 11(b)) of the recording and reproducing device. Regarding the material for the center hub 24, stainless steel, for example, SUS430 is used because the center hub 24 is often installed into the rotating spindle 25 magnetically.
For the shape of the center hub 24, the one with an outward flange 24b formed on the upper edge thereof shown in FIG. 10(b) is widely used. The outward flange 24b is mounted on one side of the transparent substrate 21, i.e. on the side whereon the recording medium layer 22 is formed, by an adhesive agent, such as epoxy adhesive agent, silicone adhesive agent, ultraviolet hardening resin and double-sided adhesive tape.
FIGS. 11(a)(b) shows the recording and reproducing device for recording and reproducing information on the magneto-optical disk using the CSS method.
The magneto-optical disk of FIGS. 10(a)(b) is magnetically secured to the rotating spindle 25, and a flying head 26 is disposed above the magneto-optical disk.
In the mean time, an optical head 29 having an optical system unit is disposed below the magneto-optical disk. Since the optical head 29 needs to move with the flying head 26 in a radial direction of the magneto-optical disk, a supporting base 28 is connected to the optical head 29. An objective lens 29a of the optical head 29 is located opposite the undersurface of a magnetic field generating member 26b of the flying head 26.
Based on the above arrangement, the following provisionally deduces a range where the flying head 26 is allowed to perform sliding action in the CSS method.
If a 3.5-in. (86-mm) diameter substrate which is most demanded as a consumer product is employed as the transparent substrate 21, the recording area where the guide tracks and the guide address grooves 21a are formed occupies the area between the radii of about 22 mm and 40 mm of the substrate. In order to ensure the magneto-optical recording characteristics in this recording area, it is preferable to for the recording medium layer 22 in the range of radius r.sub.1 ' (FIG. 10(a))=20.5 mm to radius r.sub.2 '=41.5 mm.
The size of a slider 26a of the flying head 26 is determined according to the flying characteristics and the like, and a sliding surface of the slider 26a with respect to the magneto-optical disk needs to be ensured to be about 4 mm in a radial direction and about 5 mm in a circumferential direction of the magneto-optical disk.
If the above sizes are taken into consideration, it is difficult to allow the flying head 26 to slide outside of the recording area of the magneto-optical disk, and therefore the sliding area (CSS area) needs to be located on the inside of the recording area. In order to prevent losing data in the recording medium, it is preferable not to provide the CSS area outside of a radius of 20 mm if the recording area is not formed inside of radius r.sub.1 ', i.e. a radius of 20.5 mm.
Next, disallowing for the outer side face of the rotating spindle 25 making contact with the inner edge of the optical head 29, the innermost permissible CSS area is determined.
The outer diameter of the rotating spindle 25 is determined according to a motor torque and the area of a cramping zone, and it is about 21 mm (10.5 mm in radius) here. In the mean time, the optical head 29 is provided with an actuator (not shown) for actuating an objective lens 29a to execute stable servo of a light beam, and is covered with a housing for protecting optical parts from dust or the like. Therefore, the distance between the center of the beam and the inner edge of the optical head 29 is about 8.5 mm at least. According to the above-mentioned dimensions, to prevent the rotating spindle 25 from making contact with the optical head 29, the central point of the optical beam (i.e. the position of the magnetic field generating means 26b) should not be inside of a 19-mm radius of the substrate.
Therefore, the magnetic field generating member 26b is positioned in the range of a 19 mm radius to a 20 mm radius of the substrate, and thus the area where the flying head 26 executes CSS is restricted to a very limited range.
Further, as aforesaid, the stamper presser groove 21b on the transparent substrate 21 is just secondarily formed by transferring the stamper presser member for securing the stamper to the metallic mold onto the transparent substrate 21. Therefore, the stamper presser groove 21b can not be formed much inside. If the stamper presser groove 21b is formed innermost permissible position, its outer edge is located about 15 mm radius position of the substrate (the width of the stamper presser groove 21b is about 1 mm).
If the adhesive strength and the flatness of the center hub 24 to the transparent substrate 21 are taken into account, the center hub 24 needs a certain area for adhesion, and therefore the hub 24 needs to have, for example, a diameter of about 28 mm. As described above, the CSS area is also restricted by the position of the stamper presser groove 21b and the adhesive area of the center hub 24.
Therefore, the area where CSS action can be executed is only a small range of 15 mm to 20 mm in radius. As a result, the shape of the flying head 26 including the magnetic field generating men, her 26b is restricted, and the degree of freedom of the flying characteristics decreases.
The protective resin layer 23 for protecting the recording medium layer 22 is formed in the shape of a concentric circle from the outer edge of the stamper presser groove 21b. Since the flying head has a flying force due to the air flow between the flying head 26 and the surface of the protective resin layer 23, it is necessary to have the protective resin layer 23 on the portions of the recording medium layer 22, which faces the surface of the slider 26a of the flying head 26 during CSS action. In this case, if the slider 26a deviates from its regular position and a part of the slider 26a enters an inner portion of the substrate which is not covered with the protective resin layer 23, a turbulent air flow is caused, and thereby the flying characteristics are adversely affected.
Even if the slider 26a does not enter the inner portion, turbulence from the air flow may happen as the edge of the slider 26a is very close to the edge of the protective resin layer 23 as shown in FIGS. 11(a)(b), and therefore this is not an ideal state.
The protective resin layer 23 is developed to obtain environmental resistance such as waterproof properties and moisture resistance as its duty is protecting a thin film made of an alloy of rare earth elements and transition metals which is a very easily oxidizable film. However, it fails to obtain the ability to sufficiently endure slides and wear resistance against CSS action, so some problems may arise, i.e. the slider 26a sticks to the protective resin layer 23 and dust is raised due to the abrasion. A protective resin having the ability to endure slides, wear resistance and environmental resistance has being developed. In the mean time, it is confirmed that fluorine contained polymers having the excellent ability to endure slides and wear resistance can not obtain sufficient environmental resistance.
There is an idea of forming a fluorine contained polymer resin onto the protective resin layer 23, however this causes increases in the number of parts and in cost. Additionally, the affinity between two kinds of resin is insufficient, so this is not an effective method.
Further, some magneto-optical disks which can be used with a flying head are manufactured with small physical bumps and pits (hereinafter referred to as texture) on a surface thereof facing the flying head 26 so as to prevent the disk surface from sticking to the surface of the flying head 26. As shown in FIGS. 8(a)(b), the texture is given by pressing a texture tape 31 having small bumps and pits on its surface onto a magneto-optical disk 30 by the use of a tape presser roller 32 while feeding the tape 31 in the direction of arrow C and rotating the disk 30. In this case, the direction of rotation of the disk 30 is substantially parallel with the feeding direction of the tape 31, and therefore the texture is uniformly given to the surface of the disk 30 facing the flying head 26 in the direction shown by the alternate long and two short dashes line of FIG. 9.
However, in order to manufacture the magneto-optical disk 30 with the texture, the tape 31 needs to be pressed onto the disk 30 one by one, causing an increase in cost. Thus, this is not suitable for mass manufacturing. In manufacturing, unnecessary stress may be given onto the disk 30 and dust may stick on the surface, and therefore the quality of the disk 30 may be lowered.